The invention relates to a calender for treating a paper web, in particular for papers with medium up to high demands on surface smoothness and gloss.
A calender of this type is used for the final treatment of a paper web, in order that the latter is given a desired value of smoothness, gloss, thickness, bulk and the like. The use of hard and soft rolls permits the formation of soft nips, which effect a significantly more uniform densification than the hard nips formed by two hard rolls. However, the shorter service lives of the soft rolls, caused by wear, are a disadvantage, as a result of which the outlay on maintenance and the associated down times of the known calenders are high. The rapid wear of the soft rolls, especially the roughening of the same by the roughness of the paper web surface, results primarily when calendering higher-quality papers, such as in particular those having a high filler content or coated papers which, in order to be easily printable, must have a high smoothness and a high uniformity of smoothness.
EP 0 732 443 B1 discloses the practice of replacing the resilient cover of the soft rolls by a cover which is highly insensitive to marking. However, such covers are expensive, which is particularly important taking into account the low service lives of the soft rolls.
It is therefore an object of the invention to provide a calender which supplies very good calendering results and, at the same time, is cheaper to operate.
This provides a calender for treating a paper web which reduces the disadvantages of the rapid wear of the soft rolls in that, by means of a mechanical solution with greater diameters of the soft intermediate rolls with respect to the diameter of the soft end rolls, it increases the service lives of the soft intermediate rolls in order for the latter to be matched to the service lives of the soft end rolls. Down times of the calender for maintenance actions can thus be reduced, since according to the invention all the soft rolls in a calender stack have substantially the same service lives. The maintenance actions on the soft rolls are collected together as a result of the fact that they can be performed at the same time. According to the invention, it has been established that the intermediate rolls which are rolled over twice and which, on account of this higher loading, otherwise wear more rapidly as compared with the soft end rolls, which are rolled over once, wear more slowly if the diameter is enlarged. The use of less expensive roll covers therefore becomes possible again. Calenders for producing highly glossy papers become cheaper to produce and to operate. High production speeds can be realized.
The configuration according to the invention is also important because the use of soft end rolls in a roil stack is advantageous. The use of soft end rolls permits the setting of higher temperatures in the first and/or last nip. The end rolls are usually deflection controlled rolls with an internal construction which makes the setting of high temperatures more difficult. On the other hand, if the second roll is a hard roll, then it is possible for high temperatures to be run with the latter.
According to the invention, it has been established that only a relatively low increase in the diameter of the soft intermediate rolls is sufficient to match their service lives to those of the soft end rolls. The diameter of a soft intermediate roll is advantageously 1.1 to 1.5 times the diameter of a soft end roll, although the intermediate rolls are rolled over twice as compared with the soft end rolls. The enlargement of the diameter of the soft intermediate rolls in the afore-mentioned sector depends on the paper quality, in particular the stock composition, the coating color composition or the filler content of the paper web to be treated. The higher the amount of paper web constituents which cause roughness, the greater must the diameter of the soft intermediate rolls be selected.
All rolls in a roll stack preferably have their own high-performance or power drive, in order in this way to minimize the shear stresses in the individual nips. These shear stresses, which otherwise arise from the transfer of the drive power to the rolls by the paper web running into the nips and increase the wear on the soft intermediate rolls, are avoided. The enlargement of the diameter of the intermediate rolls can then be small.
It is preferable if, depending on the web running speed, the diameter of a soft intermediate roll is selected in such a way that the loadings frequency is 20 to 25 Hz.
The configuration according to the invention permits the use of the known roll loading and/or relief systems in order to be able to set specific linear loads in the individual soft nips, so that it is possible to operate with individual linear-load characteristic curves.
The configuration according to the invention moreover permits the use of any desired roll-stack configurations. For two-side treatment of a paper web, two of the roll stacks according to the invention can be connected one behind the other, the first roll stack being used to treat one side of the paper web and the other roll stack being used to treat the other side of the paper web. Following the basic principle of the calender designer, of managing with as few rolls as possible, irrespective of the paper quality in order in this way to keep the maintenance outlay and the operating costs low, two roll stacks connected one behind the other and each having five rolls, whose top and bottom end rolls are soft rolls, are preferred. In order to optimize the guidance of the paper web, when two separate roll stacks are used, these are arranged beside each other with a vertical offset, as a result of which the path length of the paper web between the two roll stacks is shortened and the threading of the paper web is simplified. Undesired changes in the paper web can be avoided.
Alternatively, for two-side treatment of the paper web, the at least one roll stack can have six or eight rolls, the top and bottom end rolls being soft rolls and a reversing nip being formed between two soft intermediate rolls.
Further refinements of the invention are to be taken from the following description and the subclaims.